i BYZANTINE ARCHITECTURE 13
off from the central area by the screen arcade which supported the gallery. Such a narthex has been styled a 4 structural narthex/ as forming an essential part of the central building. It occurs in several of the churches of the city (p. 114).
In domed cross churches without galleries, and in churches of the c four column ? type, neither narthex nor gallery was possible within the cross, and accordingly the narthex was added to the west end. It is usually in three bays and opens into the aisles and central area. Frequently the ends or the narthex terminate in shallow niches (p. 198). In many churches a second narthex was added (p. 166) to the first, sometimes projecting an additional bay at each end, and communicating with halls or chapels on the north or south3 or on both sides of the church (p. 128). S. Mark's at Venice presents a fine example of such an extension of the narthex.
When a church could not be sufficiently enlarged by additional narthexes, a second church was built alongside the first, and both churches were joined by a narthex which extended along the front of the two buildings. S. Mary Panachrantos (p, 128) is a good example of how a church could be thus enlarged from a simple square building into a maze of passages and domes.
The Interior.—The natural division, in height, of an early church, whether basilican or domical, was into three stories—the ground level, the gallery level, and the clearstory or vault level. In the West these structural divisions were developed into the triple composition of nave-arcade3 triforium, and clearstory. In the East, in conjunction with the dome, these divisions survive in many examples of the later period, Still, Byzantine architecture was more concerned with spaces than with lines. Large surfaces for marble, painting, or mosaic were of prime importance, and with the disappearance of the gallery the string-course marking the level of the gallery also tended to disappear. In churches with galleries, like S. Theodosia (p. 170) and S. Mary Diaconissa (p. 185), the string-courses fulfil their function, the first marking the gallery level, the second the springing of the vault. In SS. Peter and Mark (p. 193),